Abstract
The geometric and electronic structures of AnCl3 are studied computationally using scalar relativistic, hybrid density functional theory (PBE0). The An-Cl bond lengths generally decrease across the 5f series, although there is a slight lengthening from Fm-Cl to No-Cl as the metal ions display increasing M(ii) character. Covalency in the An-Cl bond is studied using a wide range of metrics drawn from the Natural Bond Orbital, Natural Resonance Theory and Quantum Theory of Atoms-in-Molecules (QTAIM) methods, including bond order, orbital composition, orbital overlap and electron density topology data. Most metrics agree that the later An-Cl bonds are less ionic than might be anticipated on the basis of trends in the first half of the series, due to energy degeneracy-driven covalency in the β spin manifold; for example, the An-Cl QTAIM delocalisation index (bond order) for MdCl3 (0.88) is almost exactly the same as for NpCl3 (0.89). By contrast, the ratio of the kinetic to potential energy densities at the An-Cl bond critical points indicates that ionicity increases across the series, suggesting that the delocalisation index measures both orbital overlap and energy degeneracy-based covalency, while the bond critical point metric gauges only the former. Recalculation of all the data using the generalised gradient approximation PBE functional finds larger energy degeneracy-driven covalency in the later actinides than using hybrid DFT. Hence, we find that conclusions concerning the covalency of the An-Cl bond are dependent not only on the metric used to evaluate it, but also on the underlying electronic structure method.
Highlights
A succinct summary of the chemical behaviour of the actinide elements might be that while the early members of the series display properties characteristic of the d transition elements the later actinides are lanthanide like, with a dominant trivalent oxidation state and ionic bonding
Natural Bond Orbital (NBO) are not formed directly from natural atomic orbital (NAO), but rather from Natural Hybrid Orbitals (NHOs) which are themselves formed from the NAOs
We began with the generally accepted wisdom that the later actinides are less covalent than their earlier congeners, but does the wide range of hybrid density functional theory (DFT)-based data presented here support this? Certainly, the An–Cl Quantum Theory of Atoms-in-Molecules (QTAIM) delocalisation indices and NBO Wiberg bond indices indicate a general decrease in bond order through the first half of the 5f series, but level off and even show a small rise to Md
Summary
A succinct summary of the chemical behaviour of the actinide elements might be that while the early members of the series display properties characteristic of the d transition elements (e.g. variable oxidation state, some degree of covalent bonding) the later actinides are lanthanide like, with a dominant trivalent oxidation state and ionic bonding. The An–Cl delocalisation indices δ(An,Cl) are shown, alongside the WBIs, and are collected in Table SI9.† Despite being calculated in rather different ways, the two bond order metrics are similar to one another, both in absolute values and in trend (R2 = 0.950 for correlation of WBI with δ(An,Cl)) for Th–Md. The marked decrease at No is probably a reflection of its longer metal–chlorine distances.
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